1. Field of the Invention
The present invention relates generally to the field of molecular biology. More specifically, the invention relates to modification of plant flowering and associated phenotypes.
2. Description of the Related Art
The transition of vegetative growth to reproductive growth in plants is controlled both by endogenous signals and environmental stimuli. Genetic and molecular dissections of flowering time mutants in Arabidopsis have identified four major pathways: photoperiod pathway, autonomous pathway, gibberellin pathway and vernalization pathway (Koornneef et al., 1991, 1998; Levy and Dean, 1998). A photoperiod promotion pathway affects flowering in long days, with mutants in this pathway flowering late under long-day (LD) conditions but not late flowering in short-day (SD) conditions.
The gibberellin pathway promotes flowering response to gibberellic acid (GA) signals and mutation of genes in the GA synthesis and signaling pathway delays flowering, especially under SD conditions. Mutants in the autonomous pathway and vernalization pathway flower late under both LD and SD conditions, showing a strong response to extended low temperature by suppression of FLOWERING LOCUS C (FLC). These different floral promotion pathways are integrated by a small set of genes, called integrators, which include FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CO (SOC1), and LEAFY (LFY), forming an intricate flowering time control network (Lee et al., 2000; Mouradov et al., 2002; Simpson and Dean, 2002).
The photoperiod pathway mediates light and temporal environmental information into flowering time regulation. It comprises three parts: photoreceptors, a circadian clock and an output pathway from the clock specific to flowering (Simpson, 2003). Light is perceived by phytochromes (PHY) A through E and cryptochromes (CRY) 1 and 2, while the duration of the day and night is measured by the circadian clock (Simpson and Dean, 2002). CIRCADIAN CLOCK ASSOCIATED1 (CCA1), TIMING OF CHLOROPHYLL A/B BINDING PROTEIN1 (TOC1) and LATE ELONGATED HYPOCOTYL (LHY) are candidate genes that have been associated with the central oscillator of the circadian clock (Mouradov et al., 2002; Hayama and Coupland, 2003).
CONSTANS (CO) plays a central role in the photoperiod response pathway by mediating between the circadian clock and the floral integrators (Suárez-López et al., 2001; Searle and Coupland, 2004). CO is a transcription factor that promotes flowering time by inducing the expression of downstream genes FT (Kardailsky et al., 1999; Kobayashi et al., 1999; Onouchi et al., 2000; Samach et al., 2000) and SOC1 (Lee et al., 2000). Mutants of co exhibit a delayed flowering response under long-day (LD) but not short-day (SD) conditions (Koornneef et al., 1991; Putterill et al., 1995). Under LD conditions CO mRNA peaks in the evening and staying high until the following dawn, whereas under SD the mRNA peaks during the night (Suárez-López et al., 2001). This temporal pattern of CO expression provides a basis for the regulation of the pathway by day length (Searle and Coupland, 2004). The elevated CO mRNA level and the subsequently elevated protein level, which is stabilized by PHYA and CRY2 at late daytime, activate FT expression to promote flowering in LD (Yanovsky and Kay, 2002; Valverde et al., 2004). It has also been shown that key regulatory genes for the photoperiodic control of flowering are conserved between Arabidopsis, a LD plant, and rice, a SD plant, but regulation of FT by CO was reversed, resulting in the suppression of flowering in rice under LD conditions (Hayama et al., 2003; Simpson, 2003).
CO is a member of an Arabidopsis gene family with 16 other members (Robson et al., 2001). The family consists of putative transcription factors with two conserved domains (Putterill et al., 1995; Robson et al., 2001; Griffiths et al., 2003). The first is a zinc finger region near the amino terminus that resembles B-boxes, which is supposed to regulate protein-protein reactions. The second is the CCT (CO, CO-like, TOC1) domain near the carboxyl terminus, which is involved in nuclear localization of the proteins (Robson et al., 2001; Griffiths et al., 2003). Based on the variation of zinc finger region, the family is divided into three subgroups: group I includes CO and COL1 to COL5 with two zinc finger boxes; group II has COL6-COL8 and COL16 with one B-box; group III includes COL9 to COL15 with one B-box and a second diverged zinc finger (Robson et al., 2001).
COL9 (At3g07650, named COL11 in the TIGR Gene Indices)) is a CO-like gene the cDNA of which contains 3 introns and 4 exons and encodes a protein of 372 amino acids. The gene belongs to group III. Its amino acid sequence shares 48.5% identity in B-box region and 62% identity in CCT domain with CO. However, the function of COL9 has been unknown and has not been analyzed in vivo.
Beside CO, COL1 and COL2 are the other two characterized genes in the family. The expression of COL1 and COL2 is also regulated by circadian clock with an expression peak at dawn, but the altered expression of COL1 and COL2 had little effect on flowering time (Ledger et al., 2001). The functions of the other members in this family are largely unknown.
While the studies to date have provided a further understanding of plant flowering generally, the function of many CO-related genes remains to be determined. Effective techniques for the lengthening of flowering time in particular have thus far been lacking. There is, therefore, a great need in the art for novel methods and compositions for delaying flowering in plants. Such methods could yield plants with significantly improved nutrition and aid farmers and consumers alike.